Why is it that you can carry a child under 3 years old on your lap without them being strapped in ?
That is the most ridiculous thing I see happening on a plane. If there is a high speed crash.....you want to see a projectile?
That is technically true, but look at it this way: there are probably lots of families with children <2 years old that would not fly if they had to pay for the extra ticket. The family would drive instead. Flying is statistically many times safer than riding in a vehicle, and I'm sure the statistics include a child sitting on their parent's lap.
When you look at it that way, the parents should be congratulated for holding the child on their lap on a plane instead of putting them in greater danger by strapping them into their DOT approved car seat in the minivan and hitting the road!
The FAA has actually used this argument when reconsidering allowing lap children on flights. They decided that not allowing lap children would push families out of planes and onto the road, and they decided to leave the rule as is for now. European airlines use a harness that fastens the child to the parent's seat belt.
As to the OP, my cousin told me that one time on a flight he was sitting next to a pilot who was deadheading, and they discussed the cellphone rule. The pilot told him that it has little to do with signal interference. It has more to do with distraction, and the fact that cell phone carriers would have trouble tracking the call because of the quick handoffs from one tower to the next. It is my opinion that if there was any chance of interference that cellphones would be confiscated at boarding.